1. Field of the Invention
The present invention relates to an alignment apparatus in IC (integrated circuit) exposure apparatus. More particularly the present invention relates to such alignment apparatus operable according to the known TTL (Through The Lens) system in which the relative position of a mask to a wafer is observed through a projection optical system in a projection exposure apparatus for projecting the mask pattern on the wafer.
2. Description of the Prior Art
According to the known projection exposure method for manufacture of highly integrated circuits such as LSI and VLSI by a printing apparatus provided with an optical lens, a mask pattern is generally projected on a wafer through an image-forming projection lens. In particular when there is used such projection exposure apparatus with which a mask pattern is projected on a wafer at a minifying power in the order of .times.1/10, the same pattern is projected on one and the same wafer many times repeatedly. For this purpose it is desirable that the operator should examine the state of alignment of the mask with the wafer every time of printing on each chip while observing the relative position of the mask to the wafer through the projection lens (according to TTL system). This alignment is commonly called step-alignment or each-alignment in the art.
With the rapid development of the technique concerning IC in these years, the line width of pattern on the wafer has become thinner and thinner. Now, it is not seldom that the line width is 1 .mu.m or less. With this tendency it has been required to improve the accuracy of step-alignment and increase the speed of the alignment. There is an increasing desire for automation of alignment. Realization of automatic alignment is desired also in order to lessen the fatigue of operators in such alignment work.
As TTL automatic step-alignment there has already been known and used such alignment method according to which a wafer alignment mark image and a reticle alignment mark image are scanned by use of ITV camera or slitwise and photo-electrically converted to detect the relative position of the wafer to the reticle.
An example of TTL automatic alignment apparatus using IVT camera according to the prior art is shown in FIG. 1A for purpose of illustration.
In IC exposure apparatus, a pattern on the reticle 1 is printed on the wafer 3 through a minifying projection lens 2. To carry out TTL automatic step-alignment, an image of alignment mark 4 on the reticle 1 and an image of alignment mark 5 on the wafer 3 have to be formed again on the light reception surface of an ITV camera 14. In this connection it is to be understood that an image of the mark 5 has once been formed on the reticle 1 through the projection lens 2.
An alignment mark image 4' of the reticle and an alignment mark image 5' of the wafer formed on the light reception surface of ITV camera 14 in this manner are shown in FIG. 1B. These alignment mark images are scanned by TV scanning lines to produce photoelectric signals by means of which the relative position of the reticle 1 to the wafer 3 is detected. Relying on the result of the detection, the reticle 1 or wafer 3 is moved to a determined relative position in which the reticle 1 and the wafer (chip) get in alignment with each other.
Light source 7, optical waveguide 8, condenser lens 9, field stop 10, half-mirror 11, objective lens 12 for alignment mark and mirror 13 constitute together an illumination system.
The above shown apparatus according to the prior art has some drawbacks.
The contrast of alignment marks obtainable in such prior art apparatus is too low to measure the alignment marks without difficulty.
For example, when an alignment mark of SiO.sub.2 is provided on a wafer of Si, the contrast between the mark and the wafer is very low because Si and SiO.sub.2 have substantially the same total reflection power. The photoelectrical signal produced from ITV is an intensity signal corresponding to the contrast then obtained. Therefore, in this case, there is a possibility that it is difficult to detect the alignment mark.
In addition, when alignment has to be carried out by means of such signal having very low S/N ratio, the accuracy of alignment can be increased up only by a long signal processing operation for integrating the signals. As a long time is required for this signal processing, the speed of alignment is reduced accordingly.
Another drawback of the prior art apparatus is found in that the form of signals is not constant and it is greatly affected by the state in which the alignment mark 5 is formed on the wafer and by the condition of the photo resist coating applied thereon. For example, the resist coating often brings about such objectionable phenomenon where the alignment mark image formed on the image-forming surface appears as a dark line or a bright line or dark and bright are inverted at the edge portion of the mark due to the interference by the resist.
Due to the above drawbacks, the prior art apparatus involves difficulty and complication in correctly discerning the alignment mark and detecting its position.